The results obtained from 3 monkeys administered oral 6MP show that peak plasma levels are achieved in an average time of 60 minutes. The average half-lives of drug absorption and elimination were 0.3 and 2.7 hr, respectively. Comparison of the dose normalized area under the plasma concentration vs. time curve (AUC) following oral and I.V. doses indicated a bioavailability (F) of approximately 5%. These data suggest that following oral administration of 6MP, a singnificant amount is metabolized by xanthine oxidase enzyme during its first-pass. These data reveal that the fraction of an oral dose OF 6MP available systemically is substantially less than previously assumed and this awarness may be useful in planning strategies against acute lymphcytic leukemia.